The Weather of April 2008

April 2008 was warmer than usual. The monthly mean temperature of 23.1 degrees was 0.6 degrees above normal. The total rainfall in the month was 255.0 millimetres, 66.5 millimetres above the normal amount. The accumulated rainfall since the beginning of the year was 373.0 millimetres, about 11 percent above the normal figure of 337.1 millimetres.

A fresh to strong northeast monsoon brought cloudy and rainy weather to Hong Kong on the first three days of the month. A moist and warm maritime airstream over the south China coast brought some fog patches to the territory on the mornings of 4 to 6 April. It stayed warm with fog patches for the next four days.

A relatively cool easterly airstream arrived the south China coastal areas on 11 April and brought cooler weather to Hong Kong. It was mainly cloudy with some mist and fog patches in the ensuring three days.

The weather turned fine and hot on 15 and 16 April. The maximum temperature rose to 30.8 degrees on 16 April, the highest of the month. With the arrival of a replenishment of the northeast monsoon, weather turned cloudy with some rain on that day.

Under the combined effect of Typhoon Neoguri and the northeast monsoon, local winds started to pick up on 18 April. Local winds became generally strong in the afternoon of 19 April when Neoguri was about 200 kilometers to the west-southwest of Hong Kong. When Neoguri approached the coast of western Guangdong, the warm southerly winds associated with Neoguri met the relatively cooler northeast monsoon and formed a warm front with severe convective activities over the coastal waters of Guangdong. The warm front moved from south to north across the coast of Guangdong and brought heavy rain to Hong Kong on that day. The total daily rainfall recorded at the Hong Kong Observatory on that day was 237.4 millimetres, the highest daily rainfall amount recorded in April since record began. After making landfall and moving further inland, Neoguri weakened rapidly. Locally, rain also eased off rapidly with just a few showers on 20 April. The weather turned mainly fine for the next couple of days.

A cold front moved across the south China coast on the evening of 22 April. The northeast monsoon behind the cold front brought slightly cooler and generally cloudy weather conditions to the territory from 23 to 30 April.

Only one tropical cyclone occurred in the western North Pacific and the South China Sea in the month.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.7. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.

Warnings and Signals issued in April 2008

Table 1.1 Tropical Cyclone Warning Signals

Name of Tropical Cyclone	Signal Number	Beginning Time		Ending Time
Name of Tropical Cyclone	Signal Number	Day/Month	HKT	Day/Month	HKT
NEOGURI	1	17 / 4	1615	18 / 4	2040
NEOGURI	3	18 / 4	2040	20 / 4	0130

Table 1.2 Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
31 / 3	2045	1 / 4	0945

Table 1.3 Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
Colour	Day/Month	HKT	Day/Month	HKT
Amber	19 / 4	1640	19 / 4	1710
Red	19 / 4	1710	19 / 4	1915
Black	19 / 4	1915	19 / 4	2130

Table 1.4 Landslip Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
19 / 4	1910	20 / 4	0220

Table 1.5 Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
12 / 4	1735	12 / 4	2145
19 / 4	1330	20 / 4	0145

Table 1.6 Fire Danger Warnings

Colour	Beginning Time		Ending Time
Colour	Day/Month	HKT	Day/Month	HKT
Yellow	6 / 4	0600	6 / 4	1800
Yellow	26 / 4	1330	27 / 4	0600

Table 1.7 Special Announcement on Flooding
in the Northern New Territories

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
19 / 4	1854	20 / 4	0130

Table 2 Figures and Departures from Normal - April 2008

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	25.5 degrees C	0.4 degree above normal
Mean Air Temperature	23.1 degrees C	0.6 degree above normal
Mean Daily Minimum Air Temperature	21.5 degrees C	0.9 degree above normal
Mean Dew Point Temperature	20.3 degrees C	0.9 degree above normal
Mean Relative Humidity	85 %	2 % above normal
Mean Cloud Amount	84 %	4 % above normal
Total Rainfall	255.0 mm	66.5 mm above normal
Number of hours of Reduced VisibilityΔ	172 hours	83.5 hours above normal§
Total Bright Sunshine Duration	86.6 hours	15.2 hours below normal
Mean Daily Global Solar Radiation	11.17 Megajoule / square metre	0.66 Megajoule below normal
Total Evaporation	76.7 mm	16.5 mm below normal

Remarks :	All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station and visibility which was observed at the Hong Kong International Airport.
Δ	The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend. Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.
* Departure from 1971 - 2000 climatological normal, except for number of hours of reduced visibility
§ Departure from mean value between 1997 and 2007

daily values of selected meteorological elements for HK for April 2008

Extract of Meteorological Observations in Hong Kong for April 2008